Method for producing trichlorosilane

ABSTRACT

The invention relates to a method for producing trichlorosilane by reacting silicon with hydrogen, silicon tetrachloride and, optionally, hydrogen chloride, whereby the silicon is provided in comminuted form, and the silicon is mixed with a catalyst during comminution.

[0001] The present invention relates to a method for producingtrichlorosilane by reacting crushed silicon with silicon tetrachloride,hydrogen and, if necessary, hydrogen chloride.

[0002] Trichlorosilane HSiCl₃ is a valuable intermediate product forexample for producing high-purity silicon, dichlorosilane H₂SiCl₂,silane SiH₄ and bonding agents.

[0003] High-purity silicon is used versatilely for electronic andphoto-voltaic purposes, e.g. in the manufacture of solar cells. Toproduce high-purity silicon, metallurgical silicon is converted, forexample, to gaseous silicon compounds, preferably trichlorosilane, thesecompounds being purified and subsequently reconverted to silicon.

[0004] Trichlorosilane is mainly produced by reacting silicon withhydrogen chloride, or silicon with silicon tetrachloride, hydrogen and,if necessary, hydrogen chloride (Ullmann's Encyclopedia of IndustrialChemistry, 5^(th) ed. (1993), Vol. A24, 4-6). As a rule, silicon isreacted with silicon tetrachloride and hydrogen in the presence ofcatalysts, and mainly copper catalysts.

[0005] As is known from DE 41 04 422 A1, silicon is reacted with silicontetrachloride and hydrogen in a fluidized bed without using pressure inthe presence of copper salts of a low, aliphatic, saturated dicarbonacid, particularly copper oxalate.

[0006] It is also known to react silicon with silicon tetrachloride,hydrogen and, if necessary, hydrogen chloride, in the presence of powdercopper (Chemical Abstracts CA 101, no. 9576d, 1984) or mixtures ofcopper metal, metal halogenides and bromides or iodides of iron,aluminum or vanadium (Chemical Abstracts CA 109, no. 57621b, 1988).

[0007] Trichlorosilane is usually produced in a fluidized bed (Ullmann'sEncyclopedia of Industrial Chemistry, 5^(th) ed. (1993), Vol. A24, 4-6).A disadvantage of methods according to the state of the art using coppercatalysts and/or catalyst mixtures containing copper is that very oftensmall catalyst particles are carried out of the fluidized bed. As aresult, the yield of the desired trichlorosilane decreases in the courseof the process and new catalyst needs to be introduced into the reactor.These catalyst losses due to catalyst being carried away causeadditional costs, particularly when copper catalyst is used, sincecopper catalyst is comparatively expensive.

[0008] Therefore the task was to provide a method for producingtrichlorosilane that is characterized by a high reaction velocity and ahigh space-time yield respectively, and without large catalystquantities being carried out undesiredly.

[0009] Surprisingly it was found that only minor losses occur due tocatalyst being carried out when crushed silicon is used and the desiredcatalyst is added during milling of the silicon, particularly ofmetallurgical silicon. When reacting such crushed silicon with hydrogen,silicon tetrachloride and, if necessary, hydrogen chloride to formtrichlorosilane, the yield in trichlorosilane remained almost the same,even during longer reaction periods.

[0010] Subject-matter of the invention is therefore a method forproducing trichlorosilane by reacting silicon with hydrogen, silicontetrachloride and, if necessary, hydrogen chloride, characterized inthat the silicon is provided in crushed form and is mixed with catalystduring crushing.

[0011] The silicon to be employed according to the invention can beproduced, for example, by milling silicon, preferably metallurgicalsilicon, in a mill together with the desired catalyst, the silicon andthe catalyst being fed into the mill together or successively.

[0012] It is also possible, however, to mill the silicon in a mill madeof a material selected such that due to attrition during milling thedesired catalyst concentration is standardized in the silicon.

[0013] It is also possible, however, to mill a mixture of silicon andcatalyst in a mill made of a material that was chosen such that due toattrition during the milling process further catalytically activematerial is introduced into the silicon.

[0014] Further, the mixture of silicon and catalyst can be milled in anyother known crushing apparatuses, e.g. a jaw crusher.

[0015] Preferably the crushing is carried out in an inert atmosphere.

[0016] Working in an inert atmosphere prevents formation of an oxidiclayer on the individual silicon particles. Such layer prevents directcontact between catalyst and silicon which would result in a poorercatalysing of the reaction with silicon tetrachloride, hydrogen and, ifnecessary, hydrogen chloride to trichlorosilane.

[0017] An inert atmosphere can be achieved, for example, by adding aninert gas during milling. Suitable inert gasses are, for example,nitrogen and/or argon.

[0018] Suitable materials for the crushing apparatus in use,particularly in case of a mill or jaw crusher, are for example copper,iron and alloys of these metals with each other or with other metals,e.g. brass or bronze. Crushing apparatuses made of other materials arealso suitable, e.g. mills with a ceramic coating such as tungstencarbide.

[0019] A suitable mill, for example, can be a roller mill or a ballmill, other types of mills can also be used.

[0020] The milling is carried out so that, for example, the resultingmilled silicon has an average grain diameter of 10 to 1000 μm,preferably of 100 to 600 μm.

[0021] Suitable catalysts are, for example, copper catalysts and/or ironcatalysts.

[0022] Suitable copper catalysts are, for example, copper, preferably inthe form of copper powder with a grain size below 100 μm, or compoundsof copper, preferably copper oxide containing copper with the oxidationnumber of 1 or copper chloride, e.g. cuprous chloride.

[0023] Suitable iron catalysts are, for example, iron, preferably in theform of iron powder with a grain size below 100 μm, or compounds ofiron, preferably iron chloride, particularly preferred ferrous chloride.

[0024] It is also possible to use mixtures of copper catalysts and/oriron catalysts with further catalytically active components. Suchcatalytically active components are, for example, metal halogenides,such as e.g. chlorides, bromides or iodides of aluminum, vanadium orantimony.

[0025] The silicon to be employed according to the invention which isprovided in milled form and is mixed with a catalyst during milling canbe pre-reacted, e.g. with hydrogen chloride, or hydrogen chloride andhydrogen, before being reacted according to the invention with hydrogen,silicon tetrachloride and, if necessary, hydrogen chloride.

[0026] Usually, the silicon used in the method according to theinvention has a concentration between 0.5 to 10 weight percent,preferably between 1 to 5 weight percent catalyst calculated as metal,said weight percent being based on the total weight of milled siliconand catalyst. It is also possible, however, to use crushed silicon witha higher catalyst concentration.

[0027] The method according to the invention can be carried out, forexample, at a pressure of 1 to 40 bar (absolute), preferably of 20 to 35bar.

[0028] The process is carried out, for example, at temperatures from 400to 800° C., preferably from 450 to 600° C.

[0029] The selection of the reactor for the reaction according to theinvention is not critical, provided that under the reaction conditionsthe reactor shows adequate stability and permits the contact of thestarting materials. The process can be carried out, for example, in afixed bed reactor, a rotary tubular kiln or a fluidized-bed reactor. Itis preferred to carry out the reaction in a fluidized-bed reactor.

[0030] The mol ratio of hydrogen to silicon tetrachloride in thereaction according to the invention can be for example 0.25:1 to 4:1. Amol ratio of 0.6:1 to 2:1 is preferred.

[0031] During the reaction according to the invention hydrogen chloridecan be added, and the amounts of hydrogen chloride can be varied over awide range. Preferably an amount of hydrogen chloride is added such thata mol ratio of silicon tetrachloride to hydrogen chloride of 1:0 to1:10, particularly preferred of 1:0.5 to 1:1, is obtained.

[0032] Preferably the method according to the invention is carried outin the presence of hydrogen chloride.

[0033] The trichlorosilane produced according to the method according tothe invention can be used, for example, for the manufacture of silaneand/or hyper-pure silicon.

[0034] Therefore the invention also relates to a method for producingsilane and/or hyper-pure silicon on the basis of trichlorosilaneobtained according to the method specified above.

[0035] Preferably the method according to the invention is integratedinto a general method for producing silane and/or hyper-pure silicon.

[0036] Particularly preferred, the method according to the invention isintegrated into a multistage general method for producing hyper-puresilicon, as specified for example in “Economics of Polysilicon Process,Osaka Titanium Co., DOE/JPL 1012122 (1985), 57-78” and comprising thefollowing steps:

[0037] a) Production of trichlorosilane;

[0038] b) Disproportionation of trichlorosilane to yield silane;

[0039] c) Purifying silane to obtain high-purity silane; and

[0040] d) Thermal decomposition of silane in a fluidized-bed reactor anddepositing of hyper-pure silicon on the silicon particles which form thefluidized bed.

[0041] It is even more particularly preferred that the method accordingto the invention be integrated into a method for producing silane and/orhyper-pure silicon comprising the following steps:

[0042] 1. Trichlorosilane synthesis according to the method according tothe invention and subsequent isolation of the produced trichlorosilaneby distillation and recycling of the unreacted silicon tetrachloride,and, if desired, the unreacted hydrogen;

[0043] 2. Disproportionation of trichlorosilane to silane and silicontetrachloride through the intermediate stages of dichlorosilane andmonochlorosilane on alkaline catalysts, preferably catalysts containingamino groups, carried out in two apparatuses or in one, and recycling ofthe produced silicon provided as a high-boiling component into the firstreaction area;

[0044] 3. Further use of the silane of the purity given after theprevious step, or purifying the silane until the purity required for theintended purpose is achieved, preferably by distillation, particularlypreferred by distillation under pressure:

[0045]  and, if necessary,

[0046] 4. Thermal decomposition of silane to obtain high-purity silicon,usually above 500° C.

[0047] Apart from thermal decomposition on electrically heatedhigh-purity silicon rods, another suitable method is the thermaldecomposition in a fluidized bed consisting of hyper-pure siliconparticles, particularly when the production of solar-grade high-puritysilicon is desired. To this aim, silane can be mixed with hydrogenand/or inert gases at a mol ratio of 1:0 to 1:10.

[0048] The method according to the invention is being explained in moredetail in the following examples, without restricting the inventive ideainsofar.

EXAMPLES Example 1 (Comparative example)

[0049] In a reactor consisting of a glass tube with a diameter of 3 cmand a height of 18 cm and an in-built glass frit, silicon of the grainsize fraction of 40-400 μm was employed after having been milled in ajaw crusher the jaws of which were provided with a tungsten carbidecoating. The milling process took approx. 5 minutes per kilogram ofsilicon used. The milled silicon was mixed with cuprous chloride. Themixture of silicon and cuprous chloride had a concentration of 3 weightpercent copper. 40 g of this mixture were heated to 500° C. and agitatedby a helical ribbon impeller. A gas mixture of hydrogen and silicontetrachloride with a mol ratio of 1.85:1 was now led through the chargefrom below. The gas velocity was 2.85 cm/s, with a residence time of thegas mixture in the silicon charge of 1.7 s. The reaction occurred at apressure of 1 bar (absolute). After 30 min the yield of trichlorosilaneamounted to approx. 5%, based on the amount of silicon tetrachlorideemployed; it decreased to 0.4% after another 30 min and then remainedconstant.

Example 2

[0050] In a reactor consisting of a glass tube with a diameter of 3 cmand a height of 18 cm and an in-built glass frit, silicon of the grainsize fraction of 40-400 μm was employed after having been milledtogether with cuprous chloride in a jaw crusher the jaws of which wereprovided with a tungsten carbide coating. The milling process tookapprox. 5 minutes per kilogram of silicon used. Subsequently the mixturecontained 3 weight percent copper. 40 g of this mixture were heated to500° C. and agitated by a helical ribbon impeller. A gas mixture ofhydrogen and silicon tetrachloride with a mol ratio of 1.85:1 was nowled through this charge from below. The gas velocity was 2.85 cm/s, witha residence time of the gas mixture in the silicon charge of 1.7 s. Thereaction occurred at a pressure of 1 bar (absolute). After 30 min theyield of trichlorosilane amounted to approx. 8%, based on the amount ofsilicon tetrachloride employed; it increased to 12.1% after another 30min and then remained constant.

1. A method for producing trichlorosilane by reacting silicon withhydrogen, silicon tetrachloride and, if necessary, hydrogen chloride,characterized in that the silicon is provided as milled silicon and ismixed with a catalyst during milling.
 2. A method according to claim 1,characterized in that the silicon and the desired catalyst are milledtogether in a mill.
 3. A method according to claim 1, characterized inthat the silicon is milled in a mill made of a material from which dueto attrition during milling the desired catalyst concentration isstandardized in the silicon.
 4. A method according to at least one ofclaims 1 to 3, characterized in that the particles of the milled siliconhave an average diameter of 10 to 1000 μm.
 5. A method according to atleast one of claims 1 to 4, characterized in that the concentration ofcatalyst, calculated as metal, that is contained in the milled silicon,is between 0.5 to 10 weight percent based on the total weight of milledsilicon and catalyst.
 6. A method according to at least one of claims 1to 5, characterized in that the concentration of catalyst, calculated asmetal, that is contained in the milled silicon is between 1 to 5 weightpercent based on the total weight of milled silicon and catalyst.
 7. Amethod according to at least one of claims 1 to 6, characterized in thatthe reaction is carried out at a pressure of 1 to 40 bar (absolute). 8.A method according to at least one of claims 1 to 7, characterized inthat the reaction is carried out at temperatures from 400 to 800° C. 9.A method according to at least one of claims 1 to 8, characterized inthat the mol ratio of hydrogen to silicon tetrachloride is 0.25:1 to4:1.
 10. A method according to at least one of claims 1 to 9,characterized in that the mol ratio of hydrogen to silicon tetrachlorideis 1:0 to 1:10.
 11. A method for producing silane and/or hyper-puresilicon, characterized in that the starting material is trichlorosilaneobtained according to claims 1 to 10.